1. Field of the Invention
The present invention relates to subsurface well completion equipment and, more particularly, to an apparatus, or valve assembly, for lifting hydrocarbons from subterranean formations with gas at high production rates. Additionally, embodiments of independent and detachable actuators are disclosed.
2. Description of the Related Art
Artificial lift systems, long known by those skilled in the art of oil well production, are used to assist in the extraction of fluids from subterranean geological formations. The most ideal well for a company concerned with the production of oil, is one that flows naturally and without assistance. Often wells drilled in new fields have this advantage. In this ideal case, the pressure of the producing formation is greater than the hydrostatic pressure of the fluid in the wellbore, allowing the well to flow without artificial lift. However, as an oil bearing formation matures, and some significant percentage of the product is recovered, a reduction in the formation pressure occurs. With this reduction in formation pressure, the hydrocarbon issuance therefrom is likewise reduced to a point where the well no longer flows without assistance, despite the presence of significant volumes of valuable product still in place in the oil bearing stratum. In wells where this type of production decrease occurs, or if the formation pressure is low from the outset, artificial lift is commonly employed to enhance the recovery of oil from the formation. This disclosure is primarily concerned with one type of artificial lift called "Gas Lift." It should be noted, however, that a variety of fluid types can be used in accordance with the invention as will readily be perceived by one of ordinary skill in the art. Accordingly, the term "gas" as used herein should not be read as limiting the fluid to gases but, instead, should be read to include any of the variety of process fluids appropriate to artificial lift systems.
Gas lift has long been known to those skilled in the art, as shown in U.S. Pat. No. 2,137,441 filed in November 1938. Other patents of some historic significance are U.S. Pat. Nos. 2,672,827, 2,679,827, 2,679,903, and 2,824,525, all commonly assigned hereto. Other, more recent developments in this field include U.S. Pat. Nos. 4,239,082, 4,360,064 of common assignment, as well as U.S. Pat. Nos. 4,295,796, 4,625,941, and 5,176,164. While these patents all contributed to furthering the art of gas lift valves in wells, recent trends in drilling and completion techniques expose and highlight long felt limitations with this matured technology.
The economic climate in the oil industry of the 1990's demands that oil producing companies produce more oil, which is now exponentially more difficult to exploit, that the oil be produced in less time, and without increasing prices to the consumer. One successful technique that is currently being employed is deviated and horizontal drilling, which more efficiently drains hydrocarbon bearing formations. This increase in production makes it necessary to use much larger production tubing sizes. For example, in years past, 23/8 inch production tubing was most common. Today, tubing sizes of offshore wells range from 41/2 to 7 inches. While much more oil can be produced from tubing this large, conventional gas lift techniques have reached or exceeded their operational limit as a result.
In order for oil to be produced utilizing gas lift, a precise volume and velocity of the gas flowing upward through the tubing must be maintained. Gas injected into the hydrostatic column of fluid decreases the column's total density and pressure gradient, allowing the well to flow. As the tubing size increases, the volume of gas required to maintain the well in a flowing condition increases as the square of the increase in tubing diameter. If the volume of the gas lifting the oil is not maintained, the produced oil falls back down the tubing, and the well suffers a condition commonly known as "loading up." If the volume of gas is too great, the cost of compression and recovery of the lift gas becomes a significant percentage of the production cost. As a result, the size of a gas injection orifice in the gas lift valve is of crucial importance to the stable operation of the well. Prior art gas lift valves employ fixed diameter orifices in a range up to 3/4 inch, which may be inadequate for optimal production in large diameter tubing. This size limitation is geometrically limited by the gas lift valve's requisite small size, and the position of its operating mechanism, which prevents a full bore through the valve for maximum flow.
Other prior art gas lift valves employ pressure responsive adjustable flow valves that automatically or manually adjust the gas flow rate through the gas lift valve to attempt to maintain a certain pressure condition within the well bore and the gas lift valve. Other gas lift valves may further employ hydraulic, electric, and electric-hydraulic actuators either integral to or separate or even detachable from the gas lift valve to remotely actuate the valve and to control the extent to which the valve is opened or closed. Co-pending U.S. application Ser. No. 08/912,150 discloses several embodiments of separate or detachable actuators, or power units, responsive to hydraulic pressure to control the size of the opening of the variable orifice valve of gas lift valves and is incorporated by reference herein the same as if set forth fully herein except to the extent the teachings are consistent.
A number of existing variable orifice adjustable gas lift valves require maintenance and balance of hydraulic pressure within the gas lift valve or valve actuator to maintain a particular flow setting of the gas lift valve orifice. Position holders have been utilized, which attempt to maintain the position of the actuator relative to the gas lift valve to attempt to mechanically assure that the setting of the gas lift valve orifice remains in the position set by the operator if conditions in the hydraulic system change slightly in use. However, such prior position holders were not designed to securely hold the gas lift valve orifice in a set position without the continued supply of hydraulic pressure.
U.S. Pat. No. 5,535,767 discloses a remotely actuated adjustable choke valve having a rotatably adjustable cam and follower arrangement axially disposed within the valve body to selectively govern the relative axial position of the valve stem of the choke valve with respect to the valve body. However, the placement of the cam and follower within the valve body may be undesirable in certain applications, for example, where increased flow through the valve body is desired. Moreover, the hydraulic actuation system used to actuate the cam and follower to select the axial position of the valve stem within the valve body is also provided within the valve body, which can provide an undesirably complex valve system and also further decrease the flow rate available through the choke valve.
Because well conditions and gas lift requirements change over time, those skilled in the art of well operations are also constantly aware of the compromise of well efficiency that must be balanced versus the cost of intervention to install the most optimal gas lift valves therein as well conditions change over time. Well intervention is expensive, most especially on prolific offshore or subsea wells, so a valve that can be utilized over the entire life of the well, and whose orifice size and subsequent flow rate can be adjusted to changing downhole conditions, is a long felt and unresolved need in the oil industry. There is also a need for a novel gas lift valve that has a gas injection orifice that is large enough to inject a volume of gas adequate to lift oil in large diameter production tubing. There is also a need for differing and novel operating mechanisms for gas lift valves that will not impede the flow of injection gas therethrough and that can be remotely set using a power source without the need for a continued supply of power to maintain the desired setting of the gas lift valve orifice.